Largest-ever antimatter observation will help refine numbers for dark matter search

In experiments conducted at Brookhaven National Laboratory in the United States, an international team of physicists has detected the heaviest “antinuclei” ever observed. These tiny, short-lived objects are composed of exotic antimatter particles.

Measurements of how often these entities are produced and their properties confirm our current understanding of the nature of antimatter and will aid the search for another mysterious type of particle – dark matter – in deep space.

The results were published on August 21 in Nature.

A missing mirror world

The idea of ​​antimatter is not yet a century old. In 1928, the British physicist Paul Dirac developed a very precise theory of the behavior of electrons that predicted something ominous: the existence of electrons with negative energy, which would have made the existence of the stable universe in which we live impossible.

Fortunately, scientists have found an alternative explanation for these “negative energy” states: antielectrons, or oppositely charged twins of the electron. Antielectrons were discovered in experiments in 1932, and since then, scientists have discovered that all fundamental particles have their own antimatter counterparts.

But this raises another question. Antielectrons, antiprotons, and antineutrons should be able to combine to form antiatoms, or even antiplanets and antigalaxies. Furthermore, our Big Bang theories suggest that equal amounts of matter and antimatter must have been created at the beginning of the universe.

But everywhere we look, we see matter, and only insignificant amounts of antimatter. Where did the antimatter go? It’s a question that has been plaguing scientists for nearly a century.

Broken atom fragments

Today’s results come from the STAR experiment, located at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the United States.

The experiment involves smashing the nuclei of heavy elements like uranium together at extremely high speeds. These collisions create tiny, intense fireballs that briefly reproduce the conditions of the universe in the first milliseconds after the Big Bang.

Each collision produces hundreds of new particles, and the STAR experiment is able to detect them all. Most of these particles are unstable, short-lived entities called pions, but occasionally more interesting particles emerge.

In the STAR detector, particles pass through a large container of gas inside a magnetic field, leaving visible tracks in their wake. By measuring the “thickness” of the tracks and the extent to which they bend under the magnetic field, scientists can determine what type of particle produced them.

Matter and antimatter have opposite charges, so their paths bend in opposite directions in the magnetic field.

“Antihyperhydrogen”

In nature, the nuclei of atoms are made up of protons and neutrons. However, it is also possible to make a so-called “hypernucleus”, in which one of the neutrons is replaced by a hyperon, a slightly heavier version of the neutron.

What the researchers detected in the STAR experiment was a hypernucleus made of antimatter, or antihypernucleus. In fact, it was the heaviest and most exotic antimatter nucleus ever observed.

More precisely, it consists of an antiproton, two antineutrons and an antihyperon, and is called antihyperhydrogen 4. Of the billions of pions produced, STAR researchers identified only 16 antihyperhydrogen 4 nuclei.

Results confirm predictions

The new study compares these new, heavier antinuclei, along with a host of other lighter antinuclei, to their counterparts in normal matter. The hypernuclei are all unstable, decaying after about a tenth of a nanosecond.

Comparing the hypernuclei with their corresponding antihypernuclei, we see that they have identical lifetimes and masses, which is exactly what one would expect from Dirac’s theory.

Existing theories also accurately predict how lighter antihypernuclei are produced more often and heavier ones more rarely.

A world of shadows too?

Antimatter also has fascinating connections to another exotic substance, dark matter. From observations, we know that dark matter permeates the universe and is five times more abundant than normal matter, but we have never been able to detect it directly.

Some theories about dark matter predict that if two dark matter particles collide, they will annihilate each other and produce an explosion of matter and antimatter particles. This would then produce antihydrogen and antihelium, and an experiment called the Alpha Magnetic Spectrometer aboard the International Space Station is watching for these phenomena.

If we observed antihelium in space, how would we know whether it was produced by dark matter or normal matter? Measurements like this one from STAR allow us to calibrate our theoretical models of how much antimatter is produced in collisions with normal matter. This latest study provides a wealth of data for this type of calibration.

Fundamental questions remain

We have learned a lot about antimatter over the past century. However, we still have not been able to answer the question of why we see so little of it in the Universe.

The STAR experiment is far from alone in trying to understand the nature of antimatter and where it goes. Work at experiments such as LHCb and Alice at the Large Hadron Collider in Switzerland will advance our understanding by looking for signs of differences in the behaviour of matter and antimatter.

Perhaps by 2032, when the centenary of the initial discovery of antimatter approaches, we will have made some progress in understanding the place of this curious mirror matter in the universe – and we will even know how it relates to the enigma of dark matter.

This article is republished from The Conversation under a Creative Commons license. Read the original article.